[M2351] Fix page size in flash IAP
[M2351] Fix page size in flash IAP

In Mbed OS, page size is program unit, which is different than FMC definition.
After fixing page size, we can pass NVSTORE test (mbed-os-features-nvstore-tests-nvstore-functionality).

c1728p9/mbed-os,betzw/mbed-os,andcor02/mbed-os,betzw/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,betzw/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,betzw/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,betzw/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,c1728p9/mbed-os,betzw/mbed-os,andcor02/mbed-os